A shock absorber is installed between the body and the tire of an automobile such as motorcycles, cars, etc., and acts to reduce the vibration of the car body caused by road surface roughness, the shaking thereof to occur in quick acceleration or sudden braking, and the like.
In the expansion and contraction movement of the shock absorber, the vibration is relaxed owing to the resistance force to be generated when a lubricating oil passes through the valve arranged inside the shock absorber. The viscosity characteristics of lubricating oil have a significant influence on the resistance force and eventually on the riding comfort of automobiles. Regarding the viscosity characteristics of lubricating oil, therefore, both the viscosity increase at low temperatures and the viscosity reduction at high temperatures are desired to be small.
Recently, sale of luxury cars has become in great demand in Mideast and Russia. In Mideast, the lubricating oil temperature in a shock absorber rises up to about 80° C., while in Russia, it lowers to about −40° C. Accordingly, the improvement of the viscosity characteristics of lubricating oil mentioned above is an important theme.
When the increase in the viscosity of lubricating oil at low temperatures is intended to be suppressed, the lubricating oil tends to vaporize readily. When the lubricating oil vaporizes, the amount of the lubricating oil inside the shock absorber decreases and therefore the damping force to be caused by the bottom valve could not be generated and the riding comfort of automobiles would be thereby extremely worsened.
On the other hand, when the viscosity index of lubricating oil is increased so as to suppress the viscosity reduction at high temperatures, the shear stability of the lubricating oil tends to worsen. In the case where the lubricating oil of the type is used, the viscosity of the lubricating oil would gradually lower owing to the actuation of shock absorber and the damping force could not be generated well and, as a result, the riding comfort of automobiles is worsened.
PTLs 1 and 2 describe a lubricating oil composition for a shock absorber using a mineral oil whose pour point is −30° C. or lower. Regarding the lubricating oil composition for a shock absorber in PTLs 1 and 2, however, the Brookfield viscosity thereof at −40° C. is more than 1,000 mPa·s (Examples), and therefore the lubricating oil composition could not sufficiently improve riding comfort at low temperatures.    PTL 1: JP 2000-109876 A    PTL 2: JP 2000-109877 A